1. Field of the Invention
This invention relates generally to piston pumps and methods of reducing vapor lock during pumping. In particular, the present invention relates to magnetically driven piston pumps capable of being used with absorption heat-pump and air conditioning systems.
2. Description of the Related Art
Recent attention has been given to the commercial viability of absorption heat-pump and air conditioning systems, and, in particular, to their use in residential and commercial heating and cooling applications. This increased attention has prompted developments in reducing the physical size of such systems, increasing the heating or cooling efficiencies of such systems, and increasing the service life of such systems. As improvements are made to the overall system, individual components are also receiving increased attention and refinements as such contribute to achieving further gains associated with the heat-pump system.
One component of heat-pump systems, the absorption system solution pump, has such a large number of operating requirements and design constraints, especially in smaller tonnage systems using ammonia/water, that few improvements have been made to it by prior artisans. Such solution pumps must be relatively small in size; be corrosion resistant, particularly to a solution of ammonia and water; hermetic; be able to provide a pressure lift of at least 300 psi; be able to pump liquid, vapor or both (and thus have a net positive suction head (NPSH) of zero); be free from wear even if exposed to abrasive particles; and ideally have a relatively long service lifetime of approximately 60,000 to 80,000 hours, using no normal lubricants. Although pumping devices are known which may provide one or more of these features or abilities, none are known which provide the complete combination of these features.
Service lifetime is one factor contributing to the commercial success of a heat pump. Service lifetime means the time period a pump should operate without maintenance or failures. When pumping devices are incorporated into larger packaged systems, such as absorption heat-pump systems, the pumping device should have a service life at least as long as the packaged system, as replacement of the pumping device often requires disassembly of the system. Competitive heat-pump systems are often expected to operate up to 20 years or 60,000 hours of operation without significant maintenance. Thus, the need exists for a pumping device which has a service life of at least 60,000 to 80,000 hours.
In addition, fluid pumps used in absorption heat-pump systems employing an ammonia and water solution are particularly susceptible to interior corrosion (or other chemical reactions) from prolonged exposure to the solution. Further, corrosion problems may arise when certain salts or other additives are placed in the ammonia and water systems to increase or decrease the range of system operating temperatures, or to operate the pumps at temperatures higher or lower than the normal 80.degree.-130.degree. F. range. Thus, the need exists for a pumping device which is relatively resistant to corrosion or other chemical reactions with the solutions of ammonia and water and potential additives.
In heat-pump systems utilizing an ammonia and water solution, the pumping device must have a net positive suction head (NPSH) equal to zero because the pump will commonly be exposed to an incoming solution at or near its boiling point. If the pressure of a liquid at the pump inlet is less than the NPSH of a normal pump, the solution will at least partially vaporize, causing destructive cavitation of the pump interior. Moreover, in the ammonia-water pumps, an NPSH of zero is necessary because the pump will be required to pump vapor along with the liquid during most of its operating lifetime. The pump must also be free from the possibility of leaks and must have high efficiency.
Piston pumps, such as the pump disclosed in U.S. Pat. No. 3,584,975, have been considered for use in absorption refrigeration systems, but most of these pumps have one or more drawbacks when they are used in heat pump systems. Many existing piston pumps are not durable enough to provide the continuous and frequent operation required in a heat pump system. For example, piston pumps are susceptible to wear and/or have parts that must be replaced or repaired periodically.
Complex manufacturing processes increase the cost of many piston pumps and make them too expensive to be used in affordable heat pump systems. In addition, many existing piston pumps undergo a condition known as vapor lock when they are used to pump liquids which are near boiling point during intake or which contain significant amounts of vapor.